Generally, hybrid vehicles denote vehicles that drive by using two or more different kinds of motive power sources. In such hybrid vehicles, hybrid electric vehicles (HEVs) denote vehicles that drive by using an engine and an electric motor as motive power sources.
The HEVs are equipped with an electric motor (hereinafter referred to as a vehicle electric motor) that receives a voltage from a battery to generate a driving power for driving a vehicle.
Such vehicle electrode motors are designed to efficiently operate in a high speed driving mode where a vehicle drives at a base speed or more, in addition to a low speed driving mode where a vehicle drives at a low speed lower than the base speed.
In the high speed driving mode, a counter electromotive force which is expressed as a multiplication of a magnetic flux of a coil wound around a stator of an electric motor and a rotating speed of a rotor of the electric motor increases in proportion to the rotating speed, and thus exceeds a voltage of a battery equipped in a vehicle.
Therefore, a design for reducing a winding number for single phase of a serially connected winding coil is needed in order for the counter electromotive force to become lower than a limited voltage of a battery in the high speed driving mode.
On the other hand, in the low speed driving mode, since a current is limited, a high counter electromotive force is needed for generating a maximum torque, and to this end, a design for increasing a winding number for single phase of a winding coil is needed.
In a coil design structure of the related art, a coil winding number cannot be changed while an electric motor is being driven. Therefore, if a coil winding number increases for satisfying the required performance (or the required torque) of an electric motor which is needed in the low speed driving mode, efficiency is reduced due to a weak field control current based on the increased coil winding number in the high speed driving mode, or because a voltage of a battery is limited, the required performance is not satisfied in the high speed driving mode.
Moreover, in the coil design structure of the related art, if a coil winding number is reduced for satisfying the required performance (or the required torque) of the electric motor which is needed in the high speed driving mode, a high winding current should be applied to the electric motor in the low speed driving mode, and thus, since copper loss “P=I2r” expressed as the square “I2” of a winding current “I” and a resistance “r” of a coil winding increases, efficiency is reduced, or the specification of a limited current cannot satisfy the required performance (or the required torque).
As described above, in the related art where a winding number of a coil wound around a stator cannot be changed when a vehicle electric motor is being driven, it is unable to satisfy both the required performance which is needed in the low speed driving mode and the required performance which is needed in the high speed driving mode.